This invention relates to an ignition timing control method for internal combustion engines and, more particularly, to a method of controlling ignition timing when starting such an engine.
A well-known method of controlling the ignition timing of an internal combustion engine includes determining a basic ignition timing conforming to operating parameters of the engine and correcting this basic ignition timing in dependence upon the temperature of the engine coolant to advance the ignition timing at low coolant temperature, thereby improving the engine driveability. This conventional method of controlling ignition timing enables the basic ignition timing to be corrected to the optimum ignition timing so that the engine will run at its best output characteristic.
In an engine equipped with a catalytic exhaust emission purifier for scrubbing toxic components from the engine exhaust gases, the catalyst will fail to be activated, with an attendant decline in scrubbing efficiency, unless the temperature of the catalyst bed rises above a predetermined value. The catalyst bed, which is heated by the engine exhaust, does not readily attain its activation temperature when the exhaust temperature is low, as it is when the engine is started. In order to obtain better emission characteristics, therefore, an effective expedient is to raise the exhaust temperature immediately after the engine is started.
To this end, an ignition timing control method proposed by, for example, Japanese Provisional Patent Publication (Kokai) No. 49-95043, teaches to elevate the exhaust temperature by applying a delay angle correction to the aforementioned optimum ignition timing. However, a problem with this method is that engine output will drop, with the risk that the engine will stall, if the optimum ignition timing is delayed when the engine temperature is low.
Another approach described in, for example, the specification of Japanese Provisional Patent Publication (Kokai) No. 56-72257, is to control the ignition timing of an internal combustion engine by applying the delay angle correction to the optimum ignition timing to raise the exhaust temperature, sensing that the catalyst bed has attained a prescribed state and then reduce the magnitude of the delay angle correction in dependence upon elapsed time measured from the moment the prescribed state is attained. With this method, however, the delay angle correction is applied even when the engine is placed in a particular operating condition following engine start, such as when the engine is rapidly accelerated to propel the vehicle forward from its standing position. Applying the correction at such time prevents the acquisition of the required engine output and makes it difficult for the vehicle to attain the desired forward acceleration. Furthermore, since the delay angle correction is applied even if the engine attains a high rotating speed, there is the threat of backfire and other problems.